Gas concentration measurements are often employed to measure and/or monitor emissions into the atmosphere. Typically, a source of emissions will result in an emission plume in the atmosphere, and the main quantity of interest is often the total amount of emission in the plume. A concentration measurement at a single point is not sufficient for this task. Attempts have been made to use multi-point measurements, combined with detailed atmospheric modeling (e.g., including a turbulence model for the atmosphere) to provide plume emission estimates. However, these approaches suffer from undesirable model complexity, and the results are often disappointing in practice.
Another known approach is to take measurements over a long period of time (e.g., tens of minutes). Averaging these measurements can reduce the effect of the stochastic variability of the propagating plume sufficiently such that fitting the results of such data averaging to a simple plume model (e.g., a Gaussian plume model) can provide adequate results in some cases. However, formulating the model for the plume requires detailed knowledge of the atmosphere, and understanding of the atmospheric flow over the neighboring terrain, including obstacles. In addition, one must also know the distance to the leak and the height of the leak above ground. Any errors in the formulation of the model can lead to an unwanted bias in the reported emission rate results.
Another approach is to directly measure the emission rate by physically enclosing the leak with an impermeable surface (such as a chamber or bag), and then measuring the rate of increase of concentration in the enclosed volume. In many cases, a known flow of clean gas is injected into the volume, such that the concentration in the enclosed volume will asymptotically approach a value which is related to the emission rate of the gas, given the injection rate of the clean gas. However, this method requires physical access to the leak (or leaks), is labor intensive, and prone to underestimation bias if not all the leaks in a given facility are identified.
Accordingly, it would be an advance in the art to alleviate these limitations of complex modeling or direct access to the leak.